1. Field of the Invention
The present invention relates to a zoom lens which selects and sets a focal length as needed within a predetermined focal length range. The present invention further relates to an improved zoom lens having a large aperture, a wide angle of view in addition to a small size, high image quality and a high variable magnification ratio. The zoom lens according to the present invention can also be used in a silver salt camera with a silver salt film but the zoom lens is preferably used in an imaging apparatus using an electronic image pickup device which converts images into digital information. In particular, the present invention relates to a zoom lens used in a digital camera, a video camera, a personal computer, a mobile computer, a cell phone, a mobile information terminal device, personal digital assistant (PDA) or the like.
2. Description of the Related Art
In recent years, there are increased needs for a digital camera with high image quality, a small size, a wide angle of view and a high variable magnification ratio or the like. Product development corresponding to these needs is necessary. Therefore, a zoom lens with a high variable magnification ratio of 5 or more is necessary as a photographic lens and in order to use it for a light receiving element whose pixel number exceeds ten million pixels, a zoom lens with high image quality, a small size, a wide angle of view and a large aperture is required.
A zoom lens where in order from an object side a first optical system or a first lens group having a positive focal length, a second optical system or a second lens group having a negative focal length, a third optical system or a third lens group having a positive focal length, a fourth optical system or a fourth lens group having a negative focal length and a fifth optical system or a fifth lens group having a positive focal length are disposed and the first optical system includes a deflection optical element such as a reflective member or the like is disclosed, for example, in JP2005-215165A, JP2006-352348A, JP2007-171456A and JP2007-3598A and so on.
In addition, when a zoom lens with high performance is adapted to a high end digital camera, resolution corresponding to an image pickup device of about at least 5 million to 10 million pixels must be obtained across the whole zoom area.
Furthermore, many users also require a photographic lens with a wide angle of view and a high variable magnification ratio. A half angle of view at a wide angle end of the room lens should preferably be set to above 38 degrees. The 38 degrees half angle of view is equal to 28 mm when converted to a focal length of a 35 mm silver salt camera (so called Leica version). On the other hand, a variable magnification ratio of 5 or more is needed.
For the above requirements, JPH8-248318A and JP2005-215165A disclose a conventional zoom lens in which a thinner shape in an optical axis direction of the camera is attained by inserting a prism within the optical system.
That is, JPH8-248318A discloses a zoom lens that has disposed in order from an object side a variable magnification system constituted from a first optical system having a positive refractive power and a second optical system having a negative refractive power, an aperture, a fixed third optical system having a positive refractive power, a fourth optical system having a positive refractive power and which is movable for adjusting a focus position at a time where the distance to an subject changes or when changing the magnification or the like.
The first optical system of the zoom lens includes in order from the object side a first concave lens, an orthogonal prism, a first convex lens, a second concave lens and a cemented lens of a second convex lens.
In addition, JP2005-215165A discloses a zoom lens that includes in order from an object side a first lens group having a positive refractive power and which is fixed when zooming, a second lens group having a negative refractive power, a third lens group having a positive refractive power, a fourth lens group having a negative refractive power, a fifth lens group having a positive refractive power in which zooming is performed by moving at least the second lens group and the fourth lens group. The first lens group of the zoom lens includes in order from the object side a single first lens having a negative refractive power, a reflective member to bond a light path by 90 degrees and at least one second lens having a positive refractive power.
However, the zoom lenses disclosed in JPH8-248318A and JP2006-215165A are well suited for video cameras. In the specific embodiments, the variable magnification ratio is 8 or more in JP2005-215165A and 7 or more in JP2005-215165A. But in contrast to such a large variable magnification ratio, the half angle of view at the wide-angle end is about 33 degrees in JPH8-248318A and about 34 degrees in JP2005-215165A so that the requirement of more than 38 degrees imposed on a digital camera is not satisfied.
In JP2005-215165A, the numerical example 2 of the second embodiment discloses an example of a zoom lens of a high magnification zoom with a zoom ratio of 7. However, in this case, an aperture and the third optical system (third lens group) have an integrated constitution so that when the angle of view is widened, the optical systems disposed after the aperture towards the object side become too large and it becomes difficult to correct the aberration. Therefore, in this example, a wide angle of view of 38 degrees or more is not realized.
In JP2005-352348A, the numerical example 3 of the third embodiment discloses an example of a zoom lens of high variable magnification and wide angle in which the zoom ratio is about 35 and the wide angle of view is above 38 degrees. However, in this case, an aperture and the third optical system (third lens group) have an integrated constitution so that the size is too large. In addition, in this case, distortion becomes exceedingly large and high performance is no longer realized.
JP2007-171456A discloses in the third embodiment a zoom lens of high variable magnification and wide angle in which the zoom ratio is about 7 and the wide angle of view is above 38 degrees. However, in this case also, an aperture and the third optical system (third lens group) are integrally disposed so that a light path can only be bent in a short side direction of an image pickup device as an image pickup surface by a reflective member as the deflection optical element. Therefore, the layout of the camera becomes difficult. Under such a constitution, in order to bend the light path in a long side or longitudinal direction, the prism as the reflective member needs to be enlarged so that the first optical system (first lens group) becomes large and it becomes difficult to correct the aberration.
In each embodiment of JP2007-3598A, it is disclosed that an interval between an aperture and the third optical system (third lens group) is decreased when changing the magnification from a short focus end to a long focus end. However, in this case, a wide angle of view of 38 degrees or more and a high variable magnification ratio of 5 or more are not realized. Furthermore, under such a constitution, in order to realize a wide angle of view and a high variable magnification ratio, the conditions described in Claim 1 of JP2007-3598A needs to be satisfied.